Max tyre pressure?

[SimonVokes]

Hi All

Just a little query that is possibly irrelevant but there are figures on tyres so there is obviously limits to stick to etc, and rightly so!
On my 'van tyre sidewall there is a max psi limit of 54, if the max weight on each axle is 850kg. My MPTLM is 1565kg, so the tyres are fine (I assume) up to a potential of 1700kg.
If I load my van to the max MPTLM must I max the tyres up to 54psi or is there a formula to work out the optimal pressures for the weight etc. In the past I have tended to make the pressures 2psi under the max. I have no reason to do this other than I don't like the thought of running on the maximum pressure.
Sorry, my mind tends to wander on a Sunday night when there's no football on!

Any views??

Simon Vokes.

 

[WoodlandsCamper]

The handbook for your van should give you tyre pressures for a defined tyre size. If you don't have that info then a good approximation is to proportion the max pressure stated on the tyre by the van MTPLM. From the figures you state (assuming you mean max weight of 850 kg per tyre and not per axle and you have a single axle van) :
1565 divided by 1700, times 54 gives 50 psi

What is the max pressure stated on the tyre sidewall, as your wording is slightly misleading?

 

[Dustydog]

Hi Simon
As tyres are so Important I for one would like to know the make and model of your caravan and the exact size and make of tyre.
It is also worth repeating checking the tyres overall condition and age.
If over 5 years old they should be changed.
My Wyoming's weight plate also shows the tyre pressures which are the same from unladen upto the MTPLM.
Models and tyres vary hence the importance of knowing the specifics.

 

[ProfJohnL]

............... In the past I have tended to make the pressures 2psi under the max. I have no reason to do this other than I don't like the thought of running on the maximum pressure...........

Hello Simon,
There tyre manufacturers will have already include a margin of over pressure safety into the tyres design, do its safe to use the maximum pressure if the load demands it.

Dont forget that tyre pressures are quoted as cold pressures, and as drive the tyres heat up. I don't know how much hotter they become but the manufacturers will have done all the necessary tests to keep them safe up to the speed rating for the tyre.

You only have to watch a TV coverage of the Formula 1 to learn how cars generate heat in their tyres.

 

[colin-yorkshire]

hi all,
I have a supplementary question, regarding tyres we all know the correct pressures are important,
but what if you change the tyres for different ones for example, say to "C" type the van manufacturer would have issued one pressure "for the standard ones" say 36psi but the tyre manufacturer has stamped on the side wall max 60psi, at the max load index but the axle weight is much lower, a quick calculation puts the theoretical pressure at 45psi,
now the question, which is the correct one, as 36psi would be under inflated 60psi over inflated for the weight but 45psi although seemingly correct is actually only 3/4 of the pressure the tyre was designed to run at. would this mean the tyres are running under inflated

 

[WoodlandsCamper]

.......although seemingly correct is actually only 3/4 of the pressure the tyre was designed to run at. would this mean the tyres are running under inflated

Just because an item has been designed to operate at a max limit there is nothing to say you have to run it at the limit. Think of a normal car - designed to run up to about 120 mph, but normally only run at 70 mph.

In the case of tyres the pressure has to be commensurate with the load applied.

 

[ProfJohnL]

Hello Colin,
In the same way manufactures quote the MAM for their cars and don't expect you to run them at that weight all the time, the tyre manufacturer will mould the maximum operating pressure on the side wall, but they don't expect it to be used at that pressure unless the tyre is loaded to its limit.

 

[colin-yorkshire]

hi, thanks guy's I kind of knew the answer but it's one of those doubts that get the grey cells working overtime while reading a thread,
I changed my tyres to "C" type early on after buying the van because the standard ones were so soft at max pressure 33psi, and even though the new ones are much stiffer still look under inflated at 42psi, I did try running them at 50psi for a while but the top cupboards emptied out onto the floor during a tow, fitting Alko shocks made no difference so I dropped them back to 42psi. the van is dead stable under tow but the tyres do get very warm after a couple of hundred miles

 

[WoodlandsCamper]

Colin

"42 psi looking under-inflated"

You said your calculation was 45 psi.. At 42 psi they may be under-inflated.

 

[JonnyG]

Colin

"42 psi looking under-inflated"

You said your calculation was 45 psi.. At 42 psi they may be under-inflated.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=196....

and why has everybody assumed the load index max kg and the max psi are linked? because they are not regardless of how many ill informed sites quote this and then other follow it blindly... have stated before max kg load index are worked at at around 42psi some even at 36 psi the max psi is just the max psi you could ever put in a tyre ..so in colin case err no the tyre is not under inflated...and all the calculations being done are wrong. but at least there not under inflated ...

 

[Dustydog]

This is getting out of hand.The OP needs to tell us his make and model of caravan and tyre size.
I've just checked my tyres. Max pressure cold is 55 psi. Bailey say working pressure is 29 psi. Draw your own conclusions.

 

[SimonVokes]

You are right Dusty. My van is a Swift Challenger 510 2007. They are actually quite poor with regards to tyre info in the handbook, but my OH never throws anything away!! Our tyre size is 205/65 R15. Only had them replaced last year.
After seeing the other posts I now have a feeling I may be running them on a too high pressure, but can't seem to find a recommended running pressure from Swift despite checking several web sites including Swifts own!
Aaaaargh, dilemma!!

 

[colin-yorkshire]

This is getting out of hand.The OP needs to tell us his make and model of caravan and tyre size.
I've just checked my tyres. Max pressure cold is 55 psi. Bailey say working pressure is 29 psi. Draw your own conclusions.

hi DD, that is only of use if the van has original spec tyres, the problem comes when the tyres are not standard,
then you need some formula to calculate the correct pressure.

I have never heard of the formula WC spoke of ie MTPLM divided by max load index times max pressure.

years ago I was told work out the percentage of the max tyre pressure against load index, and then use this percentage transposed onto the MTPLM to give the correct pressure for the weight,

I did this on my own tyres and came up with 42.25psi so rounded it down to 42psi,
WC's method produces a figure of 40psi, so less than I run them at.

 

[WoodlandsCamper]

I did this on my own tyres and came up with 42.25psi so rounded it down to 42psi,
WC's method produces a figure of 40psi, so less than I run them at.

What's 2 psi amongst friends?

I did say the method I quoted was a good approximation, I didn't say it was spot on.

 

[Dustydog]

http://www.tyresafe.org/tyre-safety/caravan-tyre-safety/load-and-inflation-tables
Hi Colin
I didn't appreciate that different tyre makes may require different pressures from original.
I no longer have the original tyres . My dealer confirmed the new tyres, same size as before but Trailermax instead of Matador, should be inflated to the quoted plated pressure.
I did find the above link which supports your and WC thoughts.
BT W my last two caravans and last two cars have all had the tyre pressures quoted on "weight" plates.
Obviously this is not an industry norm.

 

[colin-yorkshire]

I did this on my own tyres and came up with 42.25psi so rounded it down to 42psi,
WC's method produces a figure of 40psi, so less than I run them at.

What's 2 psi amongst friends?

I did say the method I quoted was a good approximation, I didn't say it was spot on.

hi WC, I agree only a small difference. just shows there is more than one way to come to the same conclusion however I am not sure my method is accurate either so there you go.
over the last few days while this thread has been running I have trawled the net looking for a definitive answer but can find none so I suppose there is always going to be some guess work involved.

 

[colin-yorkshire]

http://www.tyresafe.org/tyre-safety/caravan-tyre-safety/load-and-inflation-tables

Hi Colin
I didn't appreciate that different tyre makes may require different pressures from original.
I no longer have the original tyres . My dealer confirmed the new tyres, same size as before but Trailermax instead of Matador, should be inflated to the quoted plated pressure.
I did find the above link which supports your and WC thoughts.
BT W my last two caravans and last two cars have all had the tyre pressures quoted on "weight" plates.
Obviously this is not an industry norm.

hi Dusty, thought not it is supprising just how many differences there are between makes and types,
one thing I am sure of though after reading so much materiel on the subject Bailey could have chosen better tyres for my van as standard,

 

[Dustydog]

Hi Colin
I'll second that. The tyres on my previous Bailey failed after 18 months cracking.
The dealer replaced them foc.

 

[Sproket]

Am a bit late due to being away in the van

Maybe the OP ought to ring the tyre manufacturer like I did over the increase in tyre pressure when towing the van.